The present invention relates to a light-emitting diode (hereinafter abbreviated to “LED”) in which a fluorescent material generates fluorescence when excited by light emitted from a light-emitting element. In this specification, an LED chip itself is called a “light-emitting element” and a light-emitting device including the LED chip mounted thereon, and an optical device such as a package resin or a lens system is called a “light-emitting diode” or an “LED” as a whole.
In the related art, there is known a light-emitting diode in which a light-emitting element emitting blue light is covered with a fluorescent substance or a fluorescent substance-containing resin which generates yellow light when excited by the blue light emitted from the light-emitting element. In this light-emitting diode, the blue light emitted from the light-emitting element and the yellow light generated from the fluorescent substance excited by the blue light are mixed with each other, so that the mixed light viewed as white light can be radiated to the outside. FIG. 9 shows such a specific example of the light-emitting diode. That is, FIG. 9 is a longitudinal sectional view showing the overall configuration of the related-art light-emitting diode.
The light-emitting diode 81 has a blue light-emitting element 82, a pair of silver-plated leads 83a and 83b for supplying electric power to the blue light-emitting element 82, and a cup 83c provided in an upper end portion of one 83a of the leads. The blue light-emitting element 82 is mounted on a bottom surface of the cup 83c. Two electrodes on a surface of the blue light-emitting element 82 are bonded and electrically connected to the pair of leads 83a and 83b by two wires 84a and 84b respectively. The cup 83c is filled with a fluorescent substance 85 so that the blue light-emitting element 82 is covered with the fluorescent substance 85. The fluorescent substance 85 generates yellow light when excited by blue light. The blue light-emitting element 82, the fluorescent substance 85, and so on, are sealed with a transparent epoxy resin 86. At the same time, a radiating surface of a convex lens 87 is molded of the transparent epoxy resin 86. In this configuration, when blue light is radiated from the blue light-emitting element 82, a part of the blue light directly passing through the fluorescent substance 85, and yellow light generated from the fluorescent substance 85 excited by the remaining blue light are mixed with each other in the transparent epoxy resin 86. The mixed light is radiated externally so as to be viewed as white light.
The configuration is, however, inferior in the efficiency of externally radiating the blue light emitted from the blue light-emitting element 82, and the yellow light generated when the fluorescent substance 85 is excited. That is, only a part of blue light transmitted through the layer of the fluorescent substance 85 is radiated to the outside. Because scattering of light occurs in the fluorescent substance layer 85, the average optical path to the outside is so long that absorption loss in the fluorescent substance layer 85 becomes large. Although a part of the scattered light in the fluorescent substance 85 enters the blue light-emitting element 82 again, absorption loss in the blue light-emitting element 82 is, however, large because the refractive index of the material constituting the light-emitting element is so high that the light is hardly radiated externally and easily absorbed to the material of the light-emitting element. On the other hand, yellow excitation light is generated in the whole of the fluorescent substance 85 when the blue light passes through the fluorescent substance 85. The quantity of the generated yellow excitation light is, however, large in the proximity of the blue light-emitting element 82 which is high in density of blue light. For this reason, absorption loss of the yellow light is large in the fluorescent substance 85 and in the blue light-emitting element 82 like absorption loss of the blue light. As described above, in a structure in which light emitted from the light-emitting element and transmitted through the fluorescent substance layer, and light generated by excitation of the fluorescent substance layer are radiated externally, there is a problem that absorption loss is large.